Jerry Yang was sick and tired of dealing the stress around Yahoo, so he left of his own accord, although he probably would have been forced out shortly, Kara Swisher of All Things D reports:

"As it turns out, according to numerous sources, Yang had had enough and had finally realized that perhaps the many players in the ongoing Yahoo drama inside and outside the company had also had enough of him."

Yang's departure from Yahoo was unexpected, says Swisher. The board met this morning to talk about Yang's role, and Yahoo's communcations team was caught off guard by the move.